Neutron star field decay could impact what we know

February 26, 2007 By Miranda Marquit, PhysOrg.com in Physics / General Physics

The trend of neutron star temperature (in kilo-electron volts) versus magnetic field strength for different strongly-magnetized neutron stars. The research group interprets the general increase of temperature with field strength as strong evidence that a star&acutes temperature is controlled by heat from the magnetic field after a certain age. The solid line shows a prediction assuming that cooling by
surface emission is balanced by heating from the magnetic field. The position of the line implies an average decay time of about one million years. The diamonds and stars represent magnetars. Red symbols indicate stars younger than about 104 years which have not yet had time to cool down onto the solid line. Vertical lines represent uncertainty ranges, vertical arrows denote upper limits, and horizontal arrows denote upper limits on the field. Credit: Bennett Link

“[W]hat we have found could have profound impacts on what we know about how neutron stars evolve, how old they are and even what they are made of,” Bennett Link tells PhysOrg.com.

Until now, explains Link, a professor of physics at Montana State University in Bozeman, all evidence indicated that neutron star magnetic fields last essentially forever, except in very strongly-magnetized stars -- magnetars -- which have magnetic fields in excess of 1014 G. Link and his colleagues, José Pons, Juan Miralles and Ulrich Geppert from the Department of Applied Physics at the University of Alacant in Alacant, Spain present the findings from their study of about 30 neutron stars in a Letter published in Physical Review Letters. Their observations, and the conclusions drawn from them can be found in “Evidence for Heating of Neutron Stars by Magnetic-Field Decay.”

The major finding from the team’s work is that stars with fields in excess of about 1012 G show evidence for decay of their magnetic fields. Previously, such stars were assumed to have constant magnetic fields like stars with weaker fields. “Our work fills the gap between the lower-field stars and the magnetars,” Link says. As the energy of the field dissipates into space, the cooling of these more strongly magnetized stars is delayed.

A neutron star is a super-dense stellar remnant created from a supernova explosion. More mass than is contained in the Sun is packed into an object 20 km across. One reason these objects are so interesting is because they contain matter denser than anything we can study on Earth. “You can only study the properties of dense matter to a certain point in the laboratory,” Link explains. “But if you could figure out what’s going on in a neutron star, then you could learn more about how some of the more exotic particles that you get in particle accelerator experiments, like pions, hyperons and quarks, interact.”

“One way to approach this problem is to look at how neutron stars lose their residual heat as they age. What we have found could have profound impacts on our understanding of how neutron stars cool, how old they are and even what they are made of,” Link says.

It appears that only about five percent of neutron stars, the most strongly magnetized, undergo significant field decay; this may be why previous studies, which considered the entire neutron star population, missed the effect. Link expects that expanding the study to more stars will support the work presented by him and his colleagues in Spain. However, he points out, “expanding the sample will have to await the next generation of x-ray observatories.”

And the implications of field decay? Link points out that since most neutron star ages are estimated assuming that a star’s magnetic field is constant, field decay would change estimates of neutron star ages. “If field decay takes place over about a million years, as our analysis indicates, then what we thought was a 10 million year old star may only be 2 million years old. If we’re getting the ages wrong for some stars, our whole picture of neutron star evolution should be reconsidered.”

Age determinations are not the only thing that could change in the face of magnetic field decay. “These large field neutron stars are different from other neutron stars,” Link says. “It could be that magnetic fields in ordinary, lower-field, neutron stars decay little or not at all, due to the way the field was established at the stars’ births. More research is needed to consider the possibilities.”

“We’ve opened a new can of worms,” Link continues. “There’s a lot more to be understood about how neutron star thermal and magnetic evolution proceeds. I hope we’ve opened up new lines of discussion and new areas of research that will eventually further our understanding of neutron star cooling and composition.”